Every audience member gets a stick. One side is reflective red, one is reflective green. There are cameras at the back of the room that feed an image to a computer. The image is processed, and the output is a list of colors for each seat in the room.

You could vote with it. But, the way it’s used for a lot of its applications is to use the percentage of red showing for part of the audience to control a parameter on the screen. Split the audience in half, each one controls half of the Etch-A-Sketch.

Except, I want to provide my own graphical back ends.

Have 8 kids on one side of the room control the height of a trace that scrolls across the screen. The rest of the classoom controls a trace that starts half way across the screen, and tries to match the trace set down by the first 8 kids. (Or, on edit, have the first group drive a car on a scrolling road, have the second group try to follow them.)

The questions almost write themselves – the two groups are not the same size, yet they’re trying to match ratios. Instant proportions.

Provide numerical feedback, and the relationships between part/part and part/whole ratios, percents, and fractions becomes visually and kinesthetically apparent.

The only problem with this? It’s the one thing I’d want bigger classes for.